1. Field of the Invention
The present invention relates to aircraft engine rotor systems and, more particularly, to squeeze film damper feed and seal mechanisms as embodied in such systems.
2. Description of the Prior Art
Aircraft turbine engines when rotating in a hydrodynamic support bearing do not rotate on a fixed centerline as, e.g., an axis coincident with a centerline of a supporting journal bearing. Rather, the rotor shaft describes an orbital movement defined by two-dimensional translating movement of the rotor shaft, the orbit being a trace of eccentric movement of the rotor shaft center about the fixed journal bearing center, this orbital movement being duplicated in rolling-element bearings in which the rotor is received. The properly designed squeeze-film damper is capable of dissipating large vibrational energy at the support, thus preventing unacceptable build-up of engine loads and displacements.
A conventional squeeze film damper can be defined between an inner surface of a cylindrical journal bearing enclosure and the outer race of a rolling-element bearing assembly in which the rotor is received, roller bearings being used almost exclusively in such assemblies. The squeeze film region further can be defined by spaced apart end seals, these end seals most usually being conventional piston type ring end seals. Oil is fed into the squeeze-film region, typically through oil feed holes located at the mid-land location on the bearing enclosure (housing), and leakage paths generally exist at the interface between the seals and the housing and/or journal, and at piston ring gap (split). In some cases, housing leakage holes are placed close to the seals to ensure through-flow. The squeeze film functions to develop force equal to the imposed rotor shaft loading. The quadrature (tangential) component of this force provides damping and the radial component provides a restoring of stiffness force for rotor support. Through oil flow in the squeeze-film damper there is provided a means to transfer the heat energy generated by the squeeze-film action out of the squeeze-film region and thus prevent unacceptable temperature increase.
Conventional squeeze film dampers have a number of drawbacks. Oil feed holes into the damper create undesirable distortions of the squeeze film pressure field. This reduces the overall damper effective force, its ability to provide continuous damping, and degrades the dynamic performance of an engine rotor system. Furthermore, prediction of squeeze film forces is not reliable due to complexities of the distorted pressure field. Also and because conventional piston ring end seals with large clearances are used thereby allowing large oil flow rates, damper performance is degraded because the pressure field is locally distorted and reduced especially at the outflow end of the squeeze film region.